Dual phase magnetic material component and method of forming

ABSTRACT

A magnetic component having intermixed first and second regions, and a method of preparing that magnetic component are disclosed. The first region includes a magnetic phase and the second region includes a non-magnetic phase. The method includes mechanically masking pre-selected sections of a surface portion of the component by using a nitrogen stop-off material and heat-treating the component in a nitrogen-rich atmosphere at a temperature greater than about 900° C. Both the first and second regions are substantially free of carbon, or contain only limited amounts of carbon; and the second region includes greater than about 0.1 weight % of nitrogen.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

This invention was made with Government support under contract numberDE-EE0005573, awarded by the United States Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND

The invention relates generally to a component having multiple magneticand nonmagnetic regions, and a method of forming the same. Moreparticularly, the invention relates to a component having multiplemagnetic and nonmagnetic regions, and formation of the same bynitriding.

The need for high power density and high efficiency electrical machines(i.e. electric motors and generators) has long been prevalent for avariety of applications, particularly for hybrid and/or electric vehicletraction applications. The current trend in hybrid/electric vehicletraction motor applications is to increase rotational speeds to increasethe machine's power density, and hence reduce its mass and cost.However, it is recognized that when electrical machines are used fortraction applications in hybrid/electric vehicles, there is a cleartradeoff between power density, efficiency, and the machine's constantpower speed range—and that this tradeoff presents numerous designchallenges.

The power density of an electric machine may be increased by increasingthe machine size, improving thermal management, increasing rotor speed,or by increasing the magnetic utilization. The magnetic utilization maybe increased by using a combination of processing and alloying of arotor lamination to create a dual phase magnetic material by developinglocalized areas of high and low permeability. The localized areas ofhigh and low permeability generally reduce flux losses during rotoroperation.

A range of ferrous based soft magnetic compositions of the rotorlamination may be austenitized by a combination of processes to formregions of low permeability. This phase transformation at selectedregions is normally thermally driven in the presence of carbides in thealloy. Upon local heating, the carbides that are present at selectedlocations dissolve in the matrix and depress the martensite starttemperature, thereby aiding the stabilization of austenite regions atroom temperature. However, the presence of carbides in a magneticmicrostructure is known to increase coercivity and to lower the magneticsaturation, as compared to traditional ferrous-based magnetic steels. Adifferent method of stabilizing the austenite phase at room temperaturein intermediate regions of the soft magnet, while starting from asubstantially single phase microstructure, is desired to decrease thecoercivity. Embodiments of the present invention address these and otherneeds.

BRIEF DESCRIPTION

In accordance with one aspect of the invention, a magnetic componenthaving intermixed first and second regions is disclosed. The firstregion includes a magnetic phase and the second region includes anon-magnetic phase. Both first and second regions have a concentrationof carbon, if present, that is less than about 0.05 weight %, total. Thesecond region includes greater than about 0.4 weight % of nitrogen.

In accordance with another aspect of the invention, a method ofpreparing a magnetic component is disclosed. The method includesmechanically masking pre-selected sections of a surface portion of thecomponent by using a nitrogen stop-off material and heat-treating thecomponent in a nitrogen-rich atmosphere at a temperature greater thanabout 900° C., so as to form intermixed first and second regions in themagnetic alloy. Both first and second regions have a concentration ofcarbon, if present, that is less than about 0.05 weight %, and thesecond region includes greater than about 0.4 weight % of nitrogen.Various other features and advantages will be made apparent from thefollowing detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate preferred embodiments presently contemplated forcarrying out the invention.

FIG. 1 illustrates a dual phase magnetic component, in accordance withone embodiment of the invention;

FIG. 2 illustrates a dual phase magnetic component, in accordance withanother embodiment of the invention;

FIG. 3 illustrates an exemplary topology of a dual phase magneticcomponent, that may be obtained by using the methods described herein,in accordance with one embodiment of the invention;

FIG. 4 is a cross-sectional view of a dual phase component preparedusing the methods described herein, in accordance with one embodiment ofthe invention; and

FIG. 5 is a cross-sectional view showing the microstructure differencebetween the first and second regions of a dual phase component preparedusing the methods described herein, in accordance with one embodiment ofthe invention.

DETAILED DESCRIPTION

Different embodiments of the present invention relate to a magneticcomponent, a magnetic composition of the magnetic component, and amethod of forming the magnetic component.

In one embodiment of the invention, a magnetic component is disclosed.The term “magnetic component” as used herein may be a magnetic part ofany product, such as for example, a rotor lamination of a motor. Themagnetic component described herein has intermixed first and secondregions, where the first region includes a magnetic phase and the secondregion includes a non-magnetic phase. The “intermixed first and secondregions” hereby means that there are many first regions and secondregions that are in the vicinity of each other.

Thus, the magnetic component has dual magnetic regions with one set ofregions having a magnetic phase, and another set having a non-magneticphase. As used herein, the “magnetic phase” is a region where greaterthan about 99 volume % of the region is magnetic, and in general, wouldact as a magnetic region. Further, a “non-magnetic phase” may be theregion where greater than about 90 volume % of the region isnon-magnetic. The magnetic component as used herein is usually preparedfrom a single material. As an example, the material may be a compositemagnetic member which is formed by providing a ferromagnetic portion anda non-magnetic portion, by nitriding in a controlled atmosphere. Whenthe magnetic component is made using a single material, the negativeeffects of bonding a ferromagnetic portion and a non-magnetic portionare reduced by ensuring reliability, hermeticity, and the bond strengthof the magnetic component.

The “magnetic phase” as used herein is a material in a magnetic statehaving a relative permeability greater than 1. In one embodiment, therelative permeability of the magnetic phase of the first region of themagnetic component is greater than 100, and a saturation magnetizationis greater than 1.5 Tesla. A “non-magnetic phase” as used herein hasgreater than about 90 volume % of the material in which the permeabilityis approximately 1, and the saturation magnetization is about zero.

Austenite, also known as gamma phase iron (y-Fe), is a metallic,non-magnetic allotrope of iron or a solid solution of iron with carbon.Heating the iron, iron-based metal, or steel to a temperature at whichit changes crystal structure from ferrite to austenite is calledaustenitization. The addition of certain alloying elements, such asmanganese, nickel, and carbon, can stabilize the austenitic structureeven at room temperature. A dual phase magnetic component may be formedby stabilizing austenite at room temperature, in some regions of themagnetic component, while retaining the strongly ferromagneticmartensite or ferrite phases at some other regions of the magneticcomponent.

The presence of carbon is known to stabilize the non-magnetic austenitestructure. Earlier efforts had been directed at dissolving formedcarbides at selected regions of the magnetic component to stabilizenon-magnetic phases at those regions of the magnetic component. In oneembodiment of the present invention, a non-magnetic region of themagnetic component is formed by stabilizing a low permeability austenitestructure by the addition of nitrogen, rather than carbon. Carbides assecond phases are known to be undesirable for the magnetic component.Thus, in some embodiments of the present invention, the material formingthe magnetic component is substantially free of carbon.

However, in other embodiments of the invention, the composition maycontain a relatively small level of carbon, which can sometimes increasethe tensile strength of the magnetic region. In these embodiments, thetotal amount of carbon in the magnetic and non-magnetic regions must beless than about 0.05 weight %.

The second (non-magnetic) region includes nitrogen in a quantity thatstabilizes the austenite phase. Like carbon, as nitrogen dissolves intoa ferrous alloy, the austenite phase is stabilized. Generally, thepresence of carbides, which serve to stabilize the austenite phase uponlocal heat treatment and dissolution, is established by alloying theinitial materials with carbon in the melt. When nitrogen is used toalloy iron, the workability of the alloy is often reduced. In certainembodiments of the present application, nitriding is used aftersubstantial working of the component has been completed.

In one embodiment, a method of preparing a magnetic component isdisclosed. Thermodynamic and empirical calculations may be used topredict ferrous alloy compositions that upon the addition of nitrogen atelevated temperatures form the austenite phase. A magnetic componentusing the designed ferrous alloy composition may be formed by using thetraditional methods. In one embodiment, the formed magnetic component issubjected to selective nitriding of the near-final component, withoutthe need to substantially alter the shape and size of the formedmagnetic component after nitriding. As used herein the term “selectivenitriding” is the nitriding of some (required) regions of the magneticcomponent, without substantially altering the ferromagnetic nature ofthe nearby regions. The ferromagnetic nature of the nearby regions maybe considered to be “substantially altered”, if the average saturationmagnetization of those regions is reduced by more than about 5 percent.

The selective nitriding of the magnetic component may be attained byusing different methods of nitriding. A chemical, physical, ormechanical block may be provided to the regions of the magneticcomponent where the nitriding is not desirable to retain the magnetism.For example, a chemical composition that discourages nitrogen diffusioninto the magnetic component may be used as the “nitrogen stop-off”material at some regions. A physical method of selectively introducingthe nitrogen at selected regions, while making the nitrogen unavailablefor the other regions, may be used. A mechanical block may be able tomechanically prevent the diffusion of the nitrogen at certain regions.

Nitriding may be carried out through a solid, liquid, gaseous, or plasmaroute. In one embodiment of the present invention, elevated temperaturegas nitriding is used as the preferred method to introduce nitrogen tothe part. The elevated temperatures in this technique allow for fastdiffusion of nitrogen, providing a quick processing route. Alternately,plasma nitriding may be used for the nitrogen diffusion. In order toavoid nitrogen diffusion in those areas which are intended to stayferritic (and thus magnetic), in one embodiment, a mechanical mask orstop-off material is applied to locations where nitrogen diffusion isnot desirable. Thus, in this embodiment, the pre-selected regions thatcorrespond to the regions that are supposed to remain magnetic aremasked, using a mechanical method, e.g., a nitrogen stop-off material.As used herein a “nitrogen stop-off material” is a material that iscapable of substantially stopping the nitrogen from entering into thatregion. It is not necessary that the stop-off material itself containnitrogen.

The magnetic component that is subjected to the selective nitriding maybe exemplified in FIG. 1. The magnetic component 10 is the initialcomponent that is formed by the ferromagnetic material, in the size andshape required for the final application. The magnetic component 10 isusually formed of a composition that has a very reduced concentration ofcarbon, and may be of any shape and size. For ease of understandingherein, the magnetic component 10 is envisaged to be in a rectangularshape with the top surface 12, and the bottom surface 14.

The magnetic component 10 has a certain length (l), width (w), andthickness (t). The magnetic component 10 includes two marked regions, afirst region 20 and a second region 30. The regions are situated so thatthe masked region 20 in the surface portion of the component correspondsto the first region, and region 30 that is not masked by the nitrogenstop-off material corresponds to the second region. The first region 20is designed to be the magnetic region, and is masked, using the nitrogenstop-off material (not shown) in the component 10. The second region 30is not masked and hence allows the nitrogen to diffuse through thecomponent 10, making the regions non-magnetic. One skilled in the artwould understand that depending on the shape and size of the requiredmagnetic and non-magnetic regions, the masks may be designed ofdifferent shapes and at different surfaces.

Nitrogen may be diffused into the component 10 through gas nitriding ina nitrogen-rich atmosphere, at a pressure greater than about 0.5atmosphere, and a temperature greater than about 900 degree Celsius (°C.). Generally, the diffusion of nitrogen is expected to increase with ahigher heat-treatment temperature, and an increased equilibrium surfaceconcentration of nitrogen. The increased pressure of the gasessurrounding the magnetic component 10 often leads to an increasedsurface concentration of nitrogen. Therefore, at a given condition in apure nitrogen atmosphere, a higher than atmospheric pressure and ahigher temperature will generally provide a faster nitriding process.

In one embodiment, a nitrogen-rich atmosphere is used. In oneembodiment, the nitrogen-rich atmosphere includes more than about 90% ofnitrogen. In one embodiment, nitriding is conducted in a substantiallypure nitrogen atmosphere. The substantially pure nitrogen atmosphere maybe created by filling nitrogen in a furnace cavity after evacuating theair from the furnace cavity, and purging with nitrogen or through thecontinuous flow of nitrogen during processing. In one embodiment, anambient pressure of greater than 1 atmosphere and a temperature greaterthan about 900° C. is used for nitriding. In a further embodiment, thetemperature of nitriding is greater than 1000° C.

The nitrogen stop-off material masks may be applied to the componentdepending on the desired pattern of nitrogen diffusion (andnon-diffusion) for the component 10. For example, in FIG. 1, the mask isat the surface region corresponding to the different first regions 20,and also at the surfaces covering the thickness of the component 10.Thus, in FIG. 1, nitriding would occur only through the unmasked regions30 on the top and bottom surfaces 12, and 14, and not through thethickness t of the component. In FIG. 2, the surfaces through thethickness of the component 10 further include the masked and unmaskedregions and hence, the nitrogen diffusion occurs from the top, bottom,and side surfaces in a controlled way.

When the magnetic component is subjected to gas nitriding, the nitrogendiffuses through the component, through all faces of the component 10,including the top surface 12, bottom surface 14, and all of the unmaskedside surfaces of the component. The nitrogen during nitriding isdiffused from the surfaces into the interior portions of the component10 in the unmasked regions 30. This diffused nitrogen, in conjunctionwith the composition of the magnetic component, changes the local phasestability in those regions, and converts those regions into non-magneticaustenite. Even though the nitrogen diffuses through the surface, thenitriding parameters allow the nitrogen to diffuse further into thelength, width, and thickness of the magnetic component, through all thesurfaces of the magnetic component 10, while the masked regions 30prevent substantial nitrogen diffusion through those regions.

The nitrogen may be diffusing into the interior of the component 10through the non-masked surface regions, but depending on the pressureand temperature, and certain other parameters, the diffused nitrogen mayalso slightly undercut the surface masked regions 20, thereby diffusinginto some of the interior regions 20. Hence, the strict boundaries ofthe magnetic and non-magnetic regions in the surface portion may be morediffuse in the interior portions. As a result, the percentage ofnon-magnetic regions in the interior portion is greater than thepercentage of the non-magnetic regions in the surface portion. Thus, inone embodiment, the volume % of the second region in the interiorportion is equal to or greater than the volume % of the second region inthe surface portion. Thus, if a hypothetical line starting from thesurface of a second, nonmagnetic, region is drawn through the interiorportion in a direction perpendicular to the surface portion, it ishighly likely that the perpendicular line encounters substantially 100%of the second region. However, the same may not always be true for aline that is drawn in the same manner, starting from the surface of afirst, magnetic region. In one embodiment, an “undercut” of thenon-magnetic phase is less than about 200 micrometers.

Through empirical and thermodynamic calculations, the parameters ofnitriding may be adjusted, and the nitrogen diffusion at differentdirections may be predicted for certain regions of the magneticcomponent 10, and accordingly, the mask size and shape may be altered sothat the end product obtained is approximately the desired outcome ofthe nitriding process.

The under-cutting of the non-magnetic phase may be less in the thinnermagnetic components. In thinner components, the majority of the nitrogendiffuses through the top and bottom surfaces 12, and 14 respectively(FIG. 2), and the amount of undercut may be reduced. Additionally, lesstime is required to achieve through-thickness nitrogen penetration, thusreducing the amount of time in which additional nitrogen diffuses belowthe mask, creating the undercuts. In one embodiment, the thickness ofthe component 10 is in a range from about 0.1 mm to about 5 mm. Thedesired pattern of the magnetic and non-magnetic regions of thiscomponent may be obtained by diffusing the nitrogen through theselectively masked top surface 12 and bottom surface 14, keeping theside surfaces of the thickness t completely masked.

Width 22 (FIG. 2) of the mask of the masked region 20 is the dimensionbetween unmasked regions 30, and may be designed as per the requirementof the magnetic component 10. In one embodiment, a width 34 of each ofthe second regions 30 in the surface is greater than about 0.5 mm. In afurther embodiment, a width of each of the second regions 30 in a planeperpendicular to the thickness t is greater than about 0.5 mm. (Aspreviously explained, the dimension “w” in FIG. 2 represents the overallwidth of the magnetic component 10.)

Nitriding of the magnetic component at designed conditions allows theintroduction of a tailored amount of nitrogen to be diffusedinterstitially into the magnetic component. In one embodiment, thesecond (unmasked) region 30 includes greater than about 0.4% ofnitrogen. The intermixing and the concentration of nitrogen is not onlylimited to the unmasked regions of the surface, but is present in boththe surface portion and the interior portion of the magnetic component10. The concentration of nitrogen in the surface portion and theinterior portion need not be uniform.

Depending on the applications, the desired magnetic region and thenon-magnetic region shapes and ratios may vary, and the diffusion ofnitrogen may be designed to satisfy these requirements. Accordingly, inone embodiment, a volume percent of the first region in the surface andinterior portions is equal to or greater than the volume percent of thesecond region in the surface and interior portions. In one embodiment,at least one of the first and second regions has an interconnectedgeometry. The “interconnected geometry” as used herein implies that aregion is connected all through the component, and is hence not isolatedcompletely from the similar regions, being surrounded by the otherregions completely.

Different topologies may be presented, having dual phase magneticmaterials, by using this technology. Some of them are described in U.S.Pat. No. 7,489,062 (Shah et al), which is incorporated herein byreference. The reference describes a number of different types ofsynchronous reluctance machines, having a stator and a rotor shaftoperationally disposed within the confines of the stator. FIG. 3 of thepresent application shows an example of a topology that could benefitfrom the dual-phase materials. The component 40 can represent a portionof the reluctance machine, including a magnetic region 42 and anon-magnetic region 44, collectively referred to as “laminated segments”below. The selectively shaped rotor 43 of the component 40 is configuredas a four-pole machine. Each pole can comprise a plurality of theaxially-extending, radially positioned (“stacked”) laminated segments45, 47, 49, and the like, which extend from each pole, terminating at acentral rotor shaft 51. As described in the Shah patent, the number ofpoles, and the number of laminations, can vary greatly, depending on theparticular design of the reluctance machine.

With continued reference to FIG. 3, the laminated segments effectivelyguide the magnetic flux into and out of the rotor 43. The magneticregions 42 constrain the path of the magnetic flux, while thenonmagnetic regions 44 ensure a relatively high density of magnetic fluxlines coming out of the surface of the rotor, and going into an air gapbetween the rotor and the stator. In manufacturing these types ofreluctance machines according to conventional techniques, magnetic andnonmagnetic laminations usually had to be assembled by variousmechanical/metalworking steps, e.g., cutting and welding. The presentinventors discovered that many of the desirable techniques could beobtained much more efficiently by the masking and nitriding processdescribed herein.

EXAMPLE

The example that follows is merely illustrative, and should not beconstrued to be any sort of limitation on the scope of the claimedinvention. Unless specified otherwise, all ingredients may becommercially available from common chemical suppliers.

With reference, to FIG. 4, an example component 50 of Fe-20Cr-5Mn (inweight %) alloy was selected and was painted with a commerciallyavailable stop-off paint 52 over half the sample. The component wasnitrided at 1150° C. in pure nitrogen at a pressure of 1 atm. FIG. 4shows the cross-section along thickness t (as shown in FIG. 1) of themasked component 50 displaying the location of the stop off coating 52used to mask the magnetic region 54 from nitriding. The region 56 wasnot masked and was exposed for nitriding. FIG. 5 generally shows thesubstantially ferritic microstructure of the magnetic region 54, and thechanged, austenite microstructure in the region 56 that is exposed tonitriding. FIG. 5 further depicts the undercut region 58 duringnitriding. Hence a line 60 drawn from the surface of the non-magneticregion through the thickness t may always encounter the non-magneticregion in the interior portion. However, a line 62 drawn from themagnetic surface region may or may not always encounter a magneticregion through the thickness, depending on the proximity of the line 62to the non-magnetic region.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal language of the claims.

The invention claimed is:
 1. A magnetic component, comprising:intermixed first and second regions formed from a single material,wherein (a) the first region comprises a magnetic phase; (b) the secondregion comprises a non-magnetic phase; (c) a concentration of carbon inboth the first and second regions is zero or less than about 0.05 weight%; and (d) the second region comprises greater than about 0.1 weight %of nitrogen.
 2. The component of claim 1, wherein the componentcomprises a surface portion and an interior portion, and the intermixedfirst and second regions are present in both the surface and interiorportions.
 3. The component of claim 2, wherein a volume % of the secondregion in the interior portion is equal to or greater than the volume %of the second region in the surface portion.
 4. The component of claim1, wherein the component has a thickness in a range from about 0.1 mm toabout 5 mm.
 5. The component of claim 4, wherein the component comprisesa surface portion, and the width of the second region at the surfaceportion is greater than about 0.5 mm.
 6. The component of claim 1,wherein at least one of the first and second regions has aninterconnected geometry.
 7. The component of claim 1, wherein a volume %of the first region in the component is greater than the volume % of thesecond region.
 8. The component of claim 1, wherein the second regioncomprises greater than about 0.4 weight % of nitrogen.
 9. The componentof claim 1, wherein the first and second regions are free of carbon. 10.A synchronous reluctance machine, comprising the magnetic component ofclaim 1.